Fixed Mobile Convergence (FMC) capabilities or services allow a subscriber to access the same or near-equivalent features over both mobile and wireline devices. Many existing wireless phones are incapable of performing SIP based Internet Protocol Multimedia Subsystem (IMS) registrations and are treated as unregistered endpoint devices by an IMS network and, therefore, cannot take advantages of advanced services provided by an IMS network.
In addition, individual endpoints behind a Public Branch eXchange (PBX), whether it is an Internet Protocol (IP) PBX or a Time Division Multiplexing (TDM) PBX, or a Class 5 End Office (EO) switch are usually not capable of performing SIP based IMS registration as well. These endpoints are also treated as unregistered endpoints by an IMS network.
Unregistered endpoint devices are unable to fully utilize feature rich services provided by an IMS network and receive inconsistent service treatment depending on the individual IMS network.